Semiconductor devices, such as field effect transistors (FET's), are familiar building blocks of integrated circuits. Field effect transistors are formed from an electrically isolated body of semiconductor material, such as silicon, defined in a semiconductor substrate and include a channel region defined in the semiconductor body between a source region and a drain region. Carrier mobility in the channel region is an important consideration because of its direct influence on device performance. For example, transistor output current and switching performance may be improved by enhancing the carrier mobility in the channel region. One approach for enhancing the carrier mobility is to strain the crystal lattice of the semiconductor material by introducing either compressive stress or tensile stress. The strain resulting from the stress alters the electronic band structure of the constituent semiconductor material. As a result, the in-plane carrier mobility is significantly increased.
Biaxial tensile strain may be induced uniformly across an entire substrate by introducing, between the bodies of semiconductor material and the substrate, an intervening layer of a material having a lattice constant greater than that of the substrate. For example, a body of biaxially strained silicon may be produced by introducing a thin layer of silicon germanium between the substrate and the semiconductor bodies. The tensile strain increases the interatomic spacing of the bodies in the plane of the substrate, which enhances electron mobility in device channel regions of n-channel field effect transistors (NFET's).
Uniaxial compressive strain may be induced locally in a silicon layer by process optimizations. Small amounts of compressive stress may be introduced by manipulating the properties of existing device structures. For example, local strain caused by the thermal expansion mismatch of silicon and shallow-trench isolation has been demonstrated to produce amounts of strain sufficient to alter device characteristics. Greater amounts of compressive stress may be introduced by, for example, depositing a silicon germanium layer only in the source and drain regions of p-channel field effect transistors (PFET's). The local introduction of the silicon germanium layer has the effect of adding compressive strain to the PFET channel region, which locally increases hole mobility.
A limitation on this approach is that such strained devices are notoriously difficult to fabricate. In particular, the use of silicon germanium layers for forming strained silicon has certain disadvantages that prevent large scale integration. Silicon germanium layers tend to introduce defects into the overlying silicon bodies, which reduces device yields. Large scale integration is further limited because it is difficult to make strained NFET's and strained PFET's on a single substrate. For example, a layer of silicon germanium deposited across the wafer is not suitable for optimizing both NFET's and PFET's. Silicon germanium also has poor thermal conductivity. Some dopants diffuse more rapidly through silicon germanium, which may influence diffusion doping profiles in source and drain regions formed in the silicon bodies. Another practical limitation is that the silicon germanium layer contributes to increasing the overall thickness of the device structure, which is being scaled downwardly in modern device designs.
What is needed, therefore, is a method of selectively introducing compressive strain into the channel region of a semiconductor body, and structures, devices and integrated circuits having a compressively strained channel region fabricated by the method.